This invention relates to acoustic dialers. Acoustic dialers are widely used for touch-tone dialing of telephone numbers by generating a pair of Dual Tone Multi-Frequency (DTMF) tones, and may be used in devices such as pagers, organizers, laptop computers, and phone cards. More particularly, this invention relates to acoustic dialers which dial telephone numbers by playing DTMF tones through a loudspeaker positioned in close proximity to a telephone microphone, whereby the microphone receives the tones played and relays the tones to a DTMF receiver which perceives the tones as if they resulted from keys being depressed on a telephone keypad.
In the prior art, acoustic dialers generate a unique pair of DTMF tones to simulate a particular key on a telephone keypad being depressed. These tones are generated in response to an individual key being manually depressed on the acoustic dialer or in response to automated dialing of a telephone number in the acoustic dialer's memory. The frequencies of these tones are determined by the particular choice of telephone key being represented and are selected based upon the key's position within the typical telephone keypad. The typical telephone keypad is arranged with three columns and four rows of keys: the first three rows representing the numbers 1 through 9, and the last row representing the star symbol(*), number 0, and pound symbol (#). Each column is assigned a column frequency which is typically 1209 Hertz (Hz) for the left column, 1336 Hz for the center column, and 1477 Hz for the right column. Similarly, each row is assigned a row frequency which is typically 697 Hz for the top row, 770 Hz for the upper-middle row, 852 Hz for the lower-middle row, and 941 Hz for the bottom row. Accordingly, selecting the top-left or 1 key will cause tones with frequencies of 1209 Hz and 697 Hz to be produced. Likewise, selecting the lower-right or pound key will cause tones with frequencies of 1477 Hz and 941 Hz to be generated.
The tones generated by acoustic dialers are decoded to indicate to the receiving apparatus the key being selected at the acoustic dialer. Typically, two banks of filters are utilized to determine the two frequencies being generated--and, accordingly, the key being depressed. One bank is used to represent the keypad's columns, and the other bank is used to represent the keypad's rows. The center frequencies of the column and row filters match those of the column and row tones generated by the acoustic dialer. Thus, when the 1 key of the acoustic dialer is selected, a pair of tones is generated and the receiver filters with center frequencies of 1209 Hz (from the column bank) and 697 Hz (from the row bank) have subtantial output.
For tone pairs to be properly decoded, the decoding logic requires substantial output from one filter in each bank and minimal output from all other filters. Tone pairs are rejected if more than one filter of each bank has a substantial output. In this way, talking, singing, and other background noises are prevented from being recognized as tone pairs produced by dialing. This rejection mechanism, however, may also reject valid tone pairs when non-linear characteristics which provide interfering tones are present in the signal path between the acoustic dialer and tone pair decoder. Typically, non-linear characteristics are introduced into a signal path by acoustic dialer loudspeakers and telephone microphones (especially older carbon microphones), but may also be attributed to other system non-linearities.
Interfering tones are typically sum or difference tones which produce a substantial signal strength at the filter outputs corresponding to column and row tones not being generated and are produced by the mixing of DTMF tones. The frequency of a sum tone can be determined by adding the values of the two frequencies being mixed and the frequency of a difference tone can be determined by subtracting the value of the lower of the frequencies from the value of the higher of the frequencies being mixed. An interfering tone is most problematic when its frequency is near the center frequency of one of the column or row filters, and its filtered signal strength approaches or exceeds the filtered signal strength of the strongest filter output in the same filter bank. With these traits, an interfering tone appears sufficiently similar to a DTMF tone that the receiver senses too many tones and thus rejects the valid tone pair.
For example, the 3 key generates a DTMF tone pair with frequencies of 1477 Hz and 697 Hz. For these tones, the sum frequency is 2174 Hz and the difference frequency is 780 Hz. The sum frequency is far away from the center frequencies of the receiver filters, and thus does not interfere. The difference frequency of 780 Hz, however, is only 10 Hz away from the upper-middle-row-filter center frequency of 770 Hz. In this situation, three filter outputs may have substantial signal strength: the 1477 Hz filter (due to the column DTMF tone); the 697 Hz filter (due to the row DTMF tone); and the 770 Hz filter (due to the interfering difference tone). Since both the 697 Hz filter and the 770 Hz filter are in the row filter bank, the decoder will reject the tone pair and thus not detect the dialing of the 3 key. Similarly, the 6 key DTMF tone frequencies can cause decoder rejection by having a difference frequency of 707 Hz which is only 10 Hz away from the top-row-filter center frequency of 697 Hz.
It is therefore an object of this invention to provide an acoustic dialer which is robust to the dialing of all keys of a telephone keypad via both linear and non-linear systems.